In continuous ink jet printing, ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. The approach for printing with these droplet streams. The approach for printing with these droplet streams is to selectively charge and deflect certain drops from their normal trajectories.
In order to selectively apply charge to the ink droplets it is necessary to control the locations the drops break-offs from the filaments to occur within a predetermined charge region, downstream from the orifice plate. Such control is effected by applying an energy signal of predetermined frequency and amplitude(s) to the ink filaments. Such filament break-up control, called stimulation, maintains uniform drop size and drop spacing, as well as controlling the drop break-off region.
A great number of different approaches have been developed to effect such stimulation of the ink filaments. Common general approaches are to impart the stimulation energy to ink in the manifold region or to apply it to the orifice plate. The optimum goal in applying stimulation energy is for each ink filament to receive signals, of exactly the same frequency and amplitude, that are precisely in phase. Such synchronous stimulation would enable precisely predictable time periods for imparting information charge and avoid any printing errors incident to improper droplet charging.
U.S. Pat. No. 4,646,104 describes a highly desirable system for achieving synchronous stimulation with a relatively short (e.g. 64 orifice) array. This system uses a rectangular solid print head body of high acoustic Q material, such as stainless steel, that is elongated in the direction normal to the locus of orifice plate attachment. That is, the length of the body in the desired predominate vibration direction is substantially greater than its other dimensions, and the ink manifold and orifice plate are located at one of the longitudinal ends of the body, normal to its longitudinal axis. The size of the print head body is selected, in view of its material composition, to exhibit a resonant frequency, in the longitudinal vibration mode, that is proximate the desired drop frequency of the ink drop streams. A pair of piezoelectric strips are mounted symmetrically on opposite sides of the body and constructed to expand and contract in the directions of the body's longitudinal axis.
The approach described above works well for short orifice arrays. However, because of the rectangular solid geometry needed to implement the longitudinal vibrational mode philosphy, the '104 patent approach has not been applied to longer orifice arrays, e.g. in the order of 4" or longer. In such longer array devices, travelling wave stimulation of the orifice plate (e.g., see U.S. Pat. No. 4,827,287) and stimulation by vibration of the ink with a transducer located in the manifold region (e.g. see U.S. Pat. Nos. 4,138,687 and 4,587,528) have been the chosen approaches. Travelling wave stimulation loses the advantages of synchronous drop break-off. Stimulation applied to ink in the manifold region involves energy transmission losses and variations and therefore is not as effective as stimulation of the filaments via orifice plate vibration. It also is complicated and expensive to construct such stimulating devices because of the need to avoid vibrational coupling to the orifice plate.